Sediment archives of coseismic deformation from regions of maximum coseismic displacement act as key datasets for the development and constraint of models of surface elevation change associated with Holocene great earthquakes. We present new lithologic and biostratigraphic investigations of a previously thought lost core from Portage, at the head of Turnagain Arm, upper Cook Inlet, which is within the zone of maximum coseismic subsidence during the AD 1964 Alaska earthquake. Quantitative analysis of diatoms preserved over six peat–silt couplets before the AD 1964 earthquake show six phases of rapid relative sea-level rise. Radiocarbon dates from each contact show that all six correlate with independently modelled ages of great earthquakes recorded in the Prince William Sound segment of the Aleutian megathrust during the last 4000 years. As with the other sites in the upper Cook Inlet, there is no evidence for an earthquake between the penultimate great earthquake, ∼880 a BP, and AD 1964 in contrast to those separately recorded at Kodiak Island and Katalla. Correlation of the six earthquakes at Portage with those at Girdwood, and no evidence for additional peat–silt couplets, support our previous interpretation that the extra three peat–silt couplets at Girdwood lacking evidence of intertidal sedimentation represent local-scale changes in environment rather than coseismic subsidence and short recurrence intervals of great earthquakes.